I think that glysophate-based herbicides like Roundup aren't supposed to survive in contact with the soil. They work by being absorbed into the plant via the leaves, then the poison is transported around the plant to kill the whole thing off. I suppose that if you're using a spray there might be a very slight risk of spraying leaves on the wrong plant by accident. You can also buy glysophate in paint-on form - the type I have used is called Tumbleweed.

Once, after an argument ith my husband, I went out into my garden in the dark with what I thought was a bugkilling spray, which was in fact Roundup. As I put it back in the shed, I realised my mistake. So I hosed the garden down for several hours and just lost some seedlings.

I would therefore advise hosing down any nearby plants straight after spraying.

The way i did it was to either paint it on the offending weeds or to put a protective barrier ie cardboard whilst spraying to ensure that i didn't inadvertently hit my precious plants. If the plants are small such as strawberries i cover these with plant pots to protect them whilst i spray...works a treat.

Another suggestion : to spray a particular weed, you can cut down a plastic bottle (of mineral water for instance), so as to obtain a cylinder. Place this cylinder over the weed, down to the soil, and spray inside the cylinder : this way, the neighbouring plants won't be sprayed.
Zephirine

It should not harm otherwise than on direct contact with a plant, but I have had surrounding plants die off, and that happened after painting it in the weed.
Painting on is definitely safer than spraying unless you want to clear an area altogether of all growth. The higher concentration - with stubborn weeds you may need to go up from the standard solution - the more so.
Useful trick is to have old cotton gloves on, with latex ones under, and finger-paint the weeds.